Epileptogenesis, the process by which a normal brain become chronically prone to seizures, is poorly understood. Many CNS insults (i.e. stroke, trauma, neurodegenerative disease) can induce epileptogenesis, yet no therapies currently exist to arrest this process. Although neuronal reorganization and alterations in brain physiology are associated with epileptogenesis, the functional consequences and relative importance of these changes to epileptogenesis, the functional consequences and relative importance of these changes to epileptogenesis and seizure genesis remain unknown. Many of the molecular, cellular and genetic mechanisms underlying neuronal reorganization and physiological alterations are likewise unknown. This information is crucial in providing a rational basis for the development of new therapies designed to disrupt epileptogenesis. Our hypothesis are that during epileptogenesis 1) GABAergic neurons undergo seizure- induced axonal sprouting, 2) the incidence of reciprocal granule cell- GABAergic interneuron synapses is increased, but individual mossy fiber synaptic inputs onto interneurons are weaker or less reliable and 3) different neuronal populations express unique gene expression patterns for axon guidance molecules associated with synaptic rearrangements. We will test these hypothesis at multiple time points during epileptogenesis using two different models of temporal lobe epilepsy. A combination of anatomical, electrophysiological, molecular biological and genetic approaches will be used. Results from these experiments will document morphological changes in GABAergic interneurons during epileptogenesis, identify and physiologically characterize novel aberrant excitatory inputs onto GABAergic interneurons in the epileptic dentate gyrus, and define genetic programs that encode the critical guidance cues regulating the synaptic reorganization associated with epilepsy. Results from the proposed study will contribute to a more detailed understanding of the regulation of the synaptic circuitry involved in epilepsy, memory and information processing in the hippocampus and provide insight for development of novel therapies to arrest epileptogenesis before chronic seizures develop.